Phenanthrene mineralization along a natural salinity gradient in an Urban Estuary, Boston Harbor, Massachusetts

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Abstract
The effect of varying salinity on phenanthrene and glutamate mineralization was examined in sediments along a natural salinity gradient in an urban tidal river. Mineralization was measured by trapping14CO2 from sediment slurries dosed with trace levels of [14C]phenanthrene or [14C]glutamate. Sediments from three sites representing three salinity regimes (0, 15, and 30%.) were mixed with filtered column water from each site. Ambient phenanthrene concentrations were also determined to calculate phenanthrene mineralization rates. Rates of phenanthrene mineralization related significantly to increasing salinity along the transect as determined by linear regression analysis. Rates ranged from 1 ng/hour/g dry sediment at the freshwater site to > 16 ng/hour/g dry sediment at the 30‰ salinity site. Glutamate mineralization also increased from the freshwater to the marine site; however, the relationship to salinity was not statistically significant. To examine the effect of salinity on mineralizing activities, individual sediments were mixed with filtered water of the other two sites. Slurries were also made with artificial seawater composed of 0, 15, or 30 g NaCl/ liter to substitute for overlying water. Rates of phenanthrene mineralization in the 0‰ ambient salinity sediments were not affected by higher salinity waters. Activities in the 15 and 30‰ ambient salinity sediments, however, were significantly inhibited by incubation with 0‰ salinity water. The inhibition, in large part, appears to be due to the decreased NaCl concentration of the water phase. Glutamate mineralization was affected in a similar manner, but not as dramatically as phenanthrene mineralization. The results suggest that phenanthrene degraders in low salinity estuarine sediments subject to salt water intrusion are tolerant to a wide range of salinities but phenanthrene degradation in brackish waters is mainly a function of obligate marine microorganisms.